The third generation partnership project (3GPP) wideband code division multiple access (W-CDMA) system is outlined in the operational scenarios for universal mobile telecommunications system (UMTS) releases R99/R4 and R5. Release 5 of the UMTS frequency division duplex (FDD) and time division duplex (TDD) modes have incorporated a feature called high speed downlink packet access (HSDPA) for improving throughput, latency and spectral efficiency in the downlink (DL). The principle of HSDPA is to schedule packet transmissions on the air interface to different mobiles as a function of their instantaneous experienced radio and service conditions in a dynamic manner (i.e., fast, every 2 ms in FDD or every 10 ms in wideband TDD, for example). The key functionalities of HSDPA in both FDD and TDD modes are (i) fast re-transmissions (Hybrid ARQ) of DL packets received in error over the air interface (Uu), (ii) fast uplink (UL) notification of DL packets received in error (Acknowledgements/Negative Acknowledgements), (iii) fast channel feedback in the UL on the DL channel state of a wireless transmit/receive unit (WTRU), and (iv) fat-pipe scheduling for efficiently servicing many users in the DL. This functionality, i.e., the fast, dynamic HSDPA packet scheduler, is located in the base station (i.e., the Node B) and operates in a rather autonomous manner from the radio network controller (RNC).
The RNC in a UMTS network has responsibility for network control and radio resource management (RRM). The RNC performs tasks such as, for example, user admission control and interference management using dynamic channel allocation (DCA) algorithms, and is thus key to ensuring reliable system operation and maximizing system efficiency. One measure of high efficiency is when the users are served or when the overall throughput is achieved.
In an FDD system, the RNC allocates a certain number of spreading codes for the usage of HSDPA data channels (HS-DSCHs) to each cell. Furthermore, in the FDD system, the HS-DSCH is transmitted over an HS transmission timing interval (TTI) length of 3 consecutive timeslots (3*0.66 ms=2 ms). The RNC communicates with the base station, noting that the spreading codes can be used for HSDPA by means of Iub/Iur signaling, and subsequently passes control on when to send DL packets in these codes to the base station. The RNC also notifies the WTRU by means of RRC signaling regarding which physical channels to listen for the HSDPA control channels, i.e., high speed shared control channels (HS-SCCHs), which in turn are used by the base station to dynamically notify WTRUs of the arrival of scheduled DL packets on its HS-DSCH. Also, the same information is sent from the RNC to base station, such that the base station is informed regarding which HS-SCCH channel a WTRU is to be alerted when HSDPA data is to be sent to the WTRU. Furthermore, the base station acts on an independent basis to determine, based on its own HSDPA scheduler, when to transmit HSDPA data to a particular WTRU.
In a TDD system, the RNC allocates a certain number of timeslots (TSs) for the usage of HSDPA data channels (HS-DSCHs) to each cell. The RNC communicates with the base station, noting that the TSs and spreading codes can be used for HSDPA by means of Iub/Iur signaling, and subsequently passes control on when to send DL packets in these TSs and codes to the base station. The RNC also notifies the WTRU by means of RRC signaling regarding which physical channels to listen for the HSDPA control channels, (i.e., high speed shared control channels (HS-SCCHs)), which in turn are used by the base station to dynamically notify WTRUs of the arrival of scheduled DL packets on its HS-DSCH. Also, the same information is sent from the RNC to base station, such that the base station is informed regarding which HS-SCCH channel a WTRU is to be alerted when HSDPA data is to be sent to the WTRU. Furthermore, the base station acts on an independent basis to determine, based on its own HSDPA scheduler, when to transmit HSDPA data to a particular WTRU.
In any CDMA system, efficient management of the resource “power” is key to keeping interference low and to maximizing the system capacity, (i.e., the number of simultaneously supported users and overall data throughput for all cells in an area).
For interference management, both FDD and TDD employ fast closed-loop (CL) power-control (PC) in the DL for the dedicated channels. Furthermore for the most common case of FDD and TDD conventional Release 99, 4 and 5 (R99, R4 and R5) dedicated channels (DCHs), CL PC operates within RNC controlled power limits. Thus, a dynamic range is pre-established at DCH setup and eventually adjusted during the life-time of the DCH by the RNC. The Node B is signaled the PC dynamic range by the RNC in the form of a maximum Tx power not to be exceeded and a minimum Tx power to be maintained because the RNC must make complex decisions to enhance the system performance. For example, a WTRU, requiring too much power, and thus frequently attaining the upper limit of the allowed dynamic range, creates over-proportional interference levels to other users in the system. The RNC may want to drop or to handover this WTRU's connection. Therefore, tight RNC control of power limits while still allowing autonomous base station operation within the established power limits is a key feature in a CDMA system operating with power-control.
For common channels of both TDD and FDD systems, tight control over the possible power settings is essential to ensuring that adequate coverage and service is available.
In R5, much more autonomy is given to the base station as compared to R99 and R4. In particular, the HS-DSCH is scheduled and sent solely under Node B responsibility. The RNC still retains semi-static control by signaling both the WTRU and the base station with spreading codes and TSs, which are to be used for the HSDPA service, and ensuring that control channels (HS-SCCH) and high speed shared information channels (HS-SICH) are used. Once this framework has been laid out, control is completely passed on to the base station, which schedules DL packet transmissions on its own.
In FDD applications, the RNC allocates in a semi-static manner a maximum amount of DL power, as a fraction of the total available base station Tx DL power, for the HSDPA service not to be exceeded in order to maintain the relatively high-level of interference created by the HSDPA channels within reasonable limits. This is implemented by signaling over the RNC/base station interfaces (Iub) when configuring DL channels in the base station. Otherwise, an HSDPA WTRU at a cell border could eventually be served by the Node B at a high HSDPA data rate and create such a high level of interference that any service in the neighboring cells would be heavily impacted, even adversely, and result in an unacceptable degradation of overall system capacity or service to non-HSDPA (R99 and R4) WTRUs. The RNC set maximum HSDPA power fraction per cell in turn indirectly determines the maximum data rate with which any given WTRU can be serviced. Another reason for the existence of such a control mechanism is that a certain amount of Node B DL Tx power needs to be reserved for non-HSDPA channels, such as pilot channels, common control channels or non-HSDPA DCHs.
A method and system for using an RNC control mechanism to establish a maximum HSDPA power level for each cell to provide HSDPA services does not exist for TDD. The only way to mitigate this at all is to dedicate certain timeslots to HS-DSCH and others timeslots to the other existing services (dedicated, shared, etc.). However, this does not allow the system to optimize the resource/power use of the WTRU by minimizing the timeslots a WTRU needs for handling the HS-DSCH channel along with the supporting channels (HS-SCCH or associated dedicated channels), since these channels cannot exist in common timeslots. This lack of RNC control is a clear drawback for reliable R5 TDD system operation and multi-cellular deployment of HSDPA enabled TDD systems in coexistence with R99/R4 non-HSDPA WTRUs and even potentially within the HSDPA WTRU itself when it needs to efficiently use the HS-DSCH along with dedicated and other control channels.
The maximum allowed Node B Tx power is configurable at cell setup by the RNC, but it does not discriminate between a base station's timeslots and applies to all of them. Furthermore, it does not differentiate between non-HSDPA and HSDPA channels.
It is desirable to have a signaling mechanism between the RNC and a plurality of base stations to provide HSDPA services without experiencing the disadvantages of known arrangements.